1. Field of the Invention
The present invention relates to a piston used for a swash plate type compressor.
2. Discussion of the Related Art
As a piston for a swash plate type compressor, there are known various types of pistons each of which has (a) a head portion which is slidably fitted in a cylinder bore formed in a cylinder block, (b) a neck portion which engages a swash plate, and (c) a connecting portion which connects the head portion and the neck portion. An example of such pistons is disclosed in JP-A-9-203378. In the swash plate type compressor piston disclosed in this publication, the head portion has a through-hole formed therethrough in a direction substantially parallel to the circumferential direction of the cylinder block. The surfaces of this through-hole which face in a direction intersecting the centerline of the piston are substantially flat or convexed towards the centerline, for easy removal of the piston from a casting mold or a forging die after the piston is formed by casting or forging.
Usually, the swash plate type compressor is lubricated by a lubricant oil in a liquid or mist phase contained or dispersed in a refrigerant gas or other gas to be compressed. The lubricant oil is delivered together with the gas into the crank chamber, for lubricating the contacting surfaces of the swash plate and the shoes, the contacting surfaces of the shoes and the pistons, and the contacting surfaces of the pistons and the cylinder bores. In some cases, the lubricant oil separated from the gas by a separator is delivered in a liquid state into the crank chamber through a rotary drive shaft by which the swash plate is rotated. This lubricant is splashed within the crank chamber and is eventually turned into a mist state. The known swash plate type compressor tend to suffer from a shortage of the lubricant oil, and an accordingly low degree of slidability of the swash plate, shoes, pistons and cylinder bore. In particular, the cylinder bores located at relatively high positions in the cylinder block and the pistons fitted therein are likely to suffer from insufficient lubrication and relatively low slidability. Since the specific gravity of the lubricant is larger than that of the gas to be compressed, the lubricant is sufficiently delivered to the cylinder bores and pistons located at relatively low positions in the cylinder block, and to the corresponding shoes. However, the lubricant oil is less likely to be sufficiently delivered to the cylinder bores, pistons and shoes which are located at the relatively high positions.